like every other kind of rot like crotchrot but with your head. Its when your up at 3 AM playing addicting games or reading celebrity twitters or watching terribly unintentionally hilarious pron. Basically, mindrot is doing something crushingly stupid while your mind slowly melts and all of your intelligence bleeds out of your ears.
Me- Yo, I was just so high, I decided to go on youtube and watch parkour accidents! Its was awesome but it was total fuckin mindrot.
My Friend- Dude you need a chick.
Me- Yeah real bad, fapping gets old.